1. Field of the Invention
The present invention relates generally to antennas and more particularly to a dual-band, RF and optical antenna with a shared aperture for optical as well as microwave communications using a single compact and highly efficient structure. The present invention also relates to the optimal communication system attributes that drive the antenna design, the RF and optical mode control logic, and the multi-channel receiver implementation that collectively support optimal operation under limited availability conditions from space to ground.
2. Description of the Related Art
There is increasing need for very high data rate communications for critical data transfer and command-control-communication systems for military reconnaissance and situational awareness from space. It is also desirable to be able to project this data directly to one or more users in a specific theater, as opposed to routing the data through Continental United States (CONUS) operations centers. The scientific community also requires progressively higher science data returns from its space missions. Toward the goal of maximizing data transfer rates, as well as total data throughput, it is reasonable to capitalize on the enormous bandwidths optical communications technology offers in order to support these missions. To compensate, however, for link losses and availability from space platforms directly to ground sites, dual-band (optical/RF) technologies are desirable because optical link availabilities even with site diversity are problematic. Microwave systems may have less bandwidth, but can serve as backups to the higher-gain optical link under adverse weather conditions. Combining the front-end apertures of each communication regime, specifically optical and RF, into a single joint terminal can save valuable platform real-estate, particularly for mass and volume constrained spacecraft.
Challenges posed by such merged (i.e., optical/RF) designs include accommodating the disparate nature of the two design regimes and the competing requirements for the highly precise optical surface quality components and the low mass necessary for space platforms. Conventionally, the design regimes (aperture diameters) for free-space optical communications and RF communications links have been somewhat dissimilar. Terrestrial laser links have been demonstrated over ranges of up to 10s of kilometers and require only a few inches of optical aperture due to the small amount of diffraction-induced beam spreading at optical wavelengths. RF apertures, on the other hand, are necessarily larger at microwave and millimeter-wave wavelengths to provide the desired gain despite being over a larger solid angle (beamwidth) relative to optical beams. Optical reflectors (mirrors) are also typically flatter (i.e., having higher f/#) than RF reflectors of similar size due to the difficulty of fabricating highly precise curved surfaces over large diameters. In turn, Cassegrain configurations are used to fold the optical path in order to make the design more compact. However, Cassegrain layouts at microwave frequencies for such relatively small apertures, in RF terms, are not practical. Therefore, a direct-fed RF design is desired which functions in conjunction with the Cassegrain optical design.